1. Field of the Invention
This invention relates to a contactless power receiving apparatus for receiving supply of power using magnetic field resonance, a power receiving method for the contactless power receiving apparatus and a contactless power supplying system which incorporates the contactless power receiving apparatus and the power receiving method.
2. Description of the Related Art
As a technique for allowing transmission of electric energy in a contactless fashion, an electromagnetic induction method and a magnetic field resonance method are available. The electromagnetic induction method and the magnetic field resonance method have such various differences as described below, and in recent years, attention is paid to energy transmission which uses the magnetic field resonance method.
FIG. 9 shows an example of a configuration of a contactless power supply system of the magnetic field resonance type wherein a power supply source and a power supply object or destination correspond in a one-by-one corresponding relationship to each other. Referring to FIG. 9, the contactless power supplying apparatus of the magnetic field resonance type shown includes a power supply source 100 and a power supply destination 200.
As shown in FIG. 9, the power supply source 100 may be, for example, a charging cradle and includes an AC (alternating-current) power supply 101, an excitation element 102, and a resonance element 103. Meanwhile, the power supply destination 200 may be a portable telephone terminal and includes a resonance element 201, an excitation element 202 and a rectification circuit 203.
Each of the excitation element 102 and the resonance element 103 of the power supply source and the resonance element 201 and the excitation element 202 of the power supply destination is formed from an air-core coil. In the inside of the power supply source 100, the excitation element 102 and the resonance element 103 are coupled strongly to each other by electromagnetic induction. Similarly, in the inside of the power supply destination 200, the resonance element 201 and the excitation element 202 are coupled strongly to each other by electromagnetic induction.
When the self resonance frequencies of the resonance element 103 in the form of an air-core coil of the power supply source 100 and the resonance element 201 in the form of an air-core coil of the power supply destination 200 coincide with each other, the resonance element 103 and the resonance element 201 are placed in a magnetic field resonance relationship, in which the coupling amount is maximum and the loss is minimum.
In particular, the contactless power supply system shown in FIG. 9 operates in the following manner. In particular, first in the power supply source, AC power of a predetermined frequency which is AC current from the AC power supply 101 is supplied to the excitation element 102, in which AC power to the resonance element 103 is induced by electromagnetic induction by the AC power. Here, the frequency of the AC power generated in the AC power supply 101 is equal to the self-resonance frequencies of the resonance element 103 of the power supply source and the resonance element 201 of the power supply destination.
As described hereinabove, the resonance element 103 of the power supply source and the resonance element 201 of the power supply destination are disposed in a relationship of magnetic field resonance. Therefore, with the resonance frequency, AC power is supplied from the resonance element 103 to the resonance element 201 in a contactless fashion.
In the power supply destination 200, the AC power from the resonance element 103 of the power supply source is accepted by the resonance element 201. The AC power from the resonance element 201 is supplied to the rectification circuit 203 through the excitation element 202 by electromagnetic induction and is converted by the rectification circuit 203 into and outputted as DC (direct current) power.
In this manner, AC power is supplied from the power supply source to the power supply destination in a contactless fashion. It is to be noted that the DC power outputted from the rectification circuit 203 is supplied, for example, to a charging circuit to which a battery is connected so that it is used to charge the battery.
The contactless power supply system wherein the power supply source and the power supply destination configured in such a manner as described above with reference to FIG. 9 correspond in a one-by-one corresponding relationship to each other has the following characteristics.
The contactless power supply system has such a relationship between the frequency of the AC power supply and the coupling amount as illustrated in FIG. 10A. As can be recognized from FIG. 10A, even if the frequency of the AC power supply is low or conversely high, the coupling amount is not high but exhibits its maximum amount only at a predetermined frequency with which a magnetic field resonance phenomenon occurs. In other words, the coupling amount exhibits frequency selectivity depending upon the magnetic field resonance.
Further, the contactless power supply system has such a relationship between the distance between the resonance elements 103 and 201 and the coupling amount as illustrated in FIG. 10B. As can be recognized from FIG. 10B, the coupling amount decreases as the distance between the resonance elements increases.
However, even if the distance between the resonance elements is small, the coupling amount is not necessarily great, but at a particular resonance frequency, the coupling amount exhibits a maximum value at a particular distance. Further, it can be recognized from FIG. 10B that a coupling amount higher than a fixed level can be assured if the distance between the resonance elements remains within a certain range.
Further, the contactless power supply system has such a relationship between the resonance frequency and the distance between the resonance elements with which a maximum coupling amount is obtained as illustrated in FIG. 10C. From FIG. 10C, it can be recognized that, where the resonance frequency is low, the distance between the resonance elements is great. Also it can be recognized that, where the resonance frequency is high, a maximum coupling amount is obtained by decreasing the distance between the resonance elements.
In a contactless power supply system of the electromagnetic induction type which is used widely at present, it is necessary for the power supplying source and the power supplying destination to share magnetic fluxes, and in order to send power efficiently, it is necessary for the power supplying source and the power supplying destination to be disposed closely to each other. Also axial registration of the power supplying source and the power supplying destination to be coupled to each other is significant.
Meanwhile, a contactless power supply system which uses a magnetic field resonance phenomenon is advantageous in that, in the contactless power supply system, power can be transmitted over a greater distance than that by the electromagnetic induction method and besides, even if the axial registration is not very good, the transmission efficiency does not drop very much.
From the foregoing, the contactless power supply system of the magnetic field resonance type and the contactless power supply system of the electromagnetic induction type have such differences as listed in FIG. 11. In particular, as seen in FIG. 11, the contactless power supply system of the magnetic field resonance type is tough against displacement between the transmission and reception coils, that is, between the resonance elements and permits a longer transmission distance.
Therefore, the contactless power supply system of the magnetic field resonance type can carry out power supply in such a manner as seen in FIG. 12. In particular, referring to FIG. 12, a plurality of power supply destinations which are portable terminals in FIG. 12 can be placed on a single power supply source which is a power supply cradle in FIG. 12 so that they are charged by the latter.
However, the plural power supply destinations or portable terminals placed on the power supply source or power supply cradle may include a power supply destination which should be charged up rapidly preferentially to the other power supply destinations or a power supply destination which may be charged up, for example, before use of the same is started the following day.
As an existing system which can charge a plurality of power supply destinations in a preferential order in this manner, a battery pack charging adapter of the contact type is disclosed in Japanese Patent Laid-Open No. 2004-207137 (hereinafter referred to as Patent Document 1).
The battery pack charging adapter disclosed in Patent Document 1 can charge a plurality of battery packs at the same time and includes preferential changeover means whose configuration is not particularly disclosed in Patent Document 1 such that it has a function of applying a priority order for charging to the battery packs connected thereto.
Further, as a contactless power supplying system, a charging apparatus for a contactless portable communication apparatus of the electromagnetic induction type is disclosed in Japanese Patent Laid-Open No. Hei 11-168837 (hereinafter referred to as Patent Document 2) although a priority order is not applied to power supply destinations.
In the charging apparatus for a contactless portable communication apparatus disclosed in Patent Document 2, in order to prevent a bad influence on a communication operation of a portable communication apparatus during charging, power supply from the charging apparatus is turned on/off based on information indicative of a timing at which communication from the portable communication apparatus is to be carried out.
With the charging apparatus for a contactless portable communication apparatus disclosed in Patent Document 2, although a priority order is not applied to power supply destinations as described above, the charging apparatus can actually control the charging state of the portable communication apparatus.